The USS events are appearing in the TSSTRACE report because you have OPTIONS(32) set in your PARMFILE. The return codes for USS events are not the same as the DRCs (Detailed Reason Codes). This means that any non-zero return codes in UNIX will be logged. It is very common to have non-zero return codes in UNIX and they should be ignored unless there is actually a failure. But if the non-zero return code allows things to work as expected then we do not worry about them.